As a method for counting particles in a sample liquid, the electric resistance method (also referred to as an impedance method) is known (patent document 1 and the like). In particle counting based on the electric resistance method, as shown in FIG. 7(a), an aperture (narrow part) 120 is provided on a flow path 100, and a pair of electrodes 130, 140 are provided sandwiching the aperture 120 in the flow direction. When particles X10 in a sample liquid M10 pass through the aperture 120, the electric resistance (or impedance) between electrodes 130, 140 changes in a pulse-shape. Therefore, for example, when a voltage is applied between the electrodes using a constant-current power supply (not shown), as shown in FIG. 7(b), the applied voltage also changes as a pulse voltage. The number of particles can be known by counting and/or measuring the number and characteristics of the pulse voltages in an arithmetic part (not shown) or the like. It is also possible to know the volume of the particle from the height Vp and width Vw of the pulse voltage.
For example, in conventional counting apparatuses that count blood cells based on the electric resistance method, as shown in FIG. 8, an aperture 210 is provided in a lower part of the side wall of a counting chamber 200 to contain a sample liquid. Through the aperture, the counting chamber is in communication with a small chamber 220 adjacent to the outer side. An electrode for the electric resistance method is provided in each of the counting chamber and the small chamber (electrode is not shown). The small chamber 220 is communicated with a suction pump 230 through a piping P10. A diluted sample liquid (blood) is injected into the counting chamber 200 as a sample liquid M11. When the suction pump 230 sucks the sample liquid M11, blood cells X11 in the sample liquid in the counting chamber pass through the aperture one by one as shown with an arrow, and blood cell counting based on the electric resistance method is performed as explained above by reference to FIGS. 7(a) and 7(b).